Sixth Expedition Crew flies to ISS but Don Thomas replaced because of "medical issues"  

Concerns over space radiation exposure grounded NASA astronaut Don Thomas from flying to the International Space Station (ISS) on the sixth Expedition Crew which launched last week, Flight International understands.

Space Shuttle Endeavour departed the Kennedy Space Center on 23 November as mission STS 113 with Thomas replaced by Don Pettit. The Port 1 Integrated Truss Assembly will also be installed on the ISS during STS 113.

Thomas was dropped from the crew in July after more than 18 months training as a result of a "medical issue that affects Thomas' long-duration flight qualification", said NASA at the time, adding that "the demanding nature of long term spaceflight requires a conservative approach to crew health issues".

Thomas has flown four Space Shuttle missions and clocked up 43 days in orbit, and STS 113 would have taken his total to over 100 days - significantly less than the top astronaut's 230 days or the world record 748 days held by a cosmonaut. It is not known if any other astronauts or cosmonauts have been grounded or retired after exceeding dosage levels.

Astronauts are treated as a radiation workers and are given protection. Dosage levels, however, are much higher than on the ground. The dosage varies according to the individual, and mission characteristics including space walk time, the altitude and inclination of the orbit, and the flight's duration. Radiation levels also vary according to the status of the outer electron belts and interplanetary proton flux, the position of the solar cycle and geomagnetic field conditions.

Thomas has flown on three microgravity sciences laboratory missions and a satellite deployment mission in 1994-97 all in 28¡, low Earth orbits, but he has not made any spacewalks.

NASA has limits for total radiation dosage exposure  accumulated over an astronaut's career. Passive meters are used on Shuttle missions but are not analysed until after the flight.

Source: Flight International